The area of the present invention is that of silicone compositions based on organofunctional polyorganosiloxanes (POS) crosslinkable by the cationic route under the effect of activation obtained via light radiation, preferably UV radiation, and/or via an electron beam and/or via thermal energy.
More specifically, the invention relates to the use of such compositions for the treatment of sheet gaskets, in particular for impregnating or varnishing cylinder head gaskets of internal combustion engines.
It relates in particular, using these compositions crosslinkable by the cationic route under UV radiation, e.g., to impregnating sheet gaskets (cylinder head gaskets) and/or to preparing antiadhesive coatings at the engine block/cylinder head interface of heat engines, the said interface preferably being the cylinder head gasket itself.
The silicone compositions used for this purpose were not until now silicone compositions crosslinkable by the cationic route under UV radiation.
The functioning of an internal combustion engine requires strong cohesion of the stationary components, thorough lubrication of the moving components and efficient cooling of the engine block assembly. The cylinder head gasket has a two-fold role; it provides for the cohesion of the engine block/cylinder head dismountable assembly and for the leaktightness of the intersecting circulations of the cooling liquid (water to which glycol and alkaline agents have been added), oil and gases in the course of combustion.
Cylinder head gaskets can be made of highly complex composite materials comprising a metal core made of finely perforated sheet metal, a board (preferably devoid of asbestos) composed of organic fibres and of inorganic fillers, crimped firing rings opening on the explosion chamber, and a kerb made of silicone elastomer to contain possible leaks onto the surface.
Cylinder head gaskets can also be composed simply of one or more metal sheets optionally covered with an NBR rubber elastomer coating or fluorinated elastomer coating of the Viton type (cf., for example, JP-A-082 09 113).
Cylinder head gaskets are generally subjected, prior to their use, to two types of treatment using silicones:
1xe2x80x94an impregnation treatment intended to fill the voids in order to render the board water-repellent and to improve its thermal behaviour,
2xe2x80x94and a varnishing treatment intended to lower the coefficient of friction of the gasket and to bring about good antiadhesiveness.
In addition to these treatments, a kerb made of silicone elastomer is also deposited, by silk-screen printing, on cylinder head gaskets.
The impregnation of cylinder head gaskets with crosslinked silicones is recommended in the case where the support is porous. This makes it possible to ensure leaktightness with regard to the various operating fluids of the engine (oils, petrol, fuel oil, cooling liquid or combustion gases). Furthermore, the impregnation provides the gasket with a degree of compressibility which it has to have during the flattening to which it is subjected when the cylinder head is tightened.
When the cylinder head is taken off, the gasket must be removed very easily without adhering to the surfaces of the cylinder head. It is therefore essential for both faces of the gasket to be antiadhesive and this is one of the essential roles of the varnishing by crosslinked silicone compositions. Varnishing with the silicone contributes to rendering leakproof but also to protecting the gasket with respect to possible damage during the operation of the engine (mechanical stress: thermal stress and/or shearing: heating/cooling).
The impregnating agents and varnishes conventionally used are compositions composed of silicone oils or resins of low viscosity comprising SiOH or SiH functional groups (U.S. Pat. No. 4,720,316; EP-A-272,282) or SiVinyl functional groups (DE-A-3,731,032; U.S. Pat. No. 4,499,135) or of mixtures of a silicone oil or resin comprising SiH functional groups and of a silicone oil or resin comprising SiVinyl functional groups (DE-A-3,718,599; EP-A-471,979; DE-A-3,544,740) capable of crosslinking at high temperature (generally greater than 100xc2x0 C.) in the presence of a metal catalyst, such as tin, titanium, sirconium or platinum salts, or of a peroxide.
Compositions based on silicone oils or resins comprising SiOH, SiH, SiVinyl or SiH/SiVinyl functional groups generating exhibit at least one of the following disadvantages:
the mixture composed of the silicone matrix and the catalyst is unstable at ambient temperature (in particular in the absence of solvent), this instability being reflected by an increase in viscosity, indeed even by a gelling, of the impregnation or varnishing bath, which requires frequent replacement of the impregnation or varnishing bath or treatment in two stages; for example, Patents U.S. Pat. No. 4,720,316 and EP-A-272,382 recommend the use of the catalyst during the manufacture of the board and then impregnation of the dried board with a silicone oil possessing SiH groups;
it is often necessary to disperse the silicone+catalyst mixture in an organic solvent in order to increase the stability and to decrease the viscosity of the said mixture; this results in the disadvantages related to the use of solvents (toxicity, provision of safety devices, of devices for recycling the solvent, and the like)
the cost price of the starting materials is high, in particular in the case of vinylated oils or resins optionally used in combination with oils or resins possessing SiH functional groups and with a platinum catalyst; these mixtures can be very stable at ambient temperature, in particular in the presence of a platinum inhibitor, but are expensive because of the use of vinylated oils or resins.
In the quest for a novel silicone composition for the treatment of cylinder head gaskets having the object of improving the abovementioned known compositions (Rhxc3x4ne-Poulenc French patent application on cylinder head gasketsxe2x80x94U.S. Pat. No. 2,697,532), provision has been made to use a silicone composition crosslinkable by hydrosilylation comprising an xcex1,xcfx89-diOH POS, a POS possessing an SiH unit of the PMHS type, and ethynylcyclohexanol, the latter compound making it possible to form resins possessing a silanol functional group; the ratio of SiH to SiOH of the POS concerned being of the order of 10/1 to 30/1 and the composition also comprising a polyaddition catalyst of the Karstedt type.
Even if such a composition effectively leads to improvement with regard to stability, reactivity and flexibility of use, it still remains the case that the crosslinking specific to this type of composition comprises a restrictive stage of heating at high temperature (160xc2x0 C.) for 5 to 10 minutes. Such a stage is particularly penalizing with regard to the productivity of an industrial process for the treatment or coating of cylinder head gaskets using silicones. It may thus be necessary, for example, to resort to the use of sophisticated and expensive equipment, such as long heating tunnels. It therefore seems that these silicone compositions, crosslinkable by polyaddition and used for the coating of cylinder head gaskets, still do not contribute an entirely satisfactory solution to the problem of carrying out silicone impregnation and treatment of cylinder head gaskets which meet the specifications of antiadhesiveness and of compressibility, as well as the requirements of productivity and of profitability for large scale production.
French Patent Application No. 2,722,203 relates to the application of a crosslinkable fluorosilicone coating to a cylinder head gasket. Such a coating is obtained from a silicone composition crosslinkable, under the effect of heat, of UV radiation or of an electron beam, by hydrosilylation. This composition comprises a vinylated fluorosilicone, a POS carrying SiH units, a platinum hydrosilylation catalyst, a condensation catalyst of the peroxide type and, optionally, a solvent of the halogenated solvent, ester or acetone type. The support to which this composition can be applied is described as preferably having a silicone nature. This coating is more specifically intended to reduce the impermeability with respect to oil of cylinder head gaskets. It is a relatively thick coating: 0.1-10 mm, which is characterized by a relatively long crosslinking time of several minutes and which has to be activated by heat for the crosslinking, even in the cases where UV radiation is used. This known coating is therefore not itself satisfactory either as regards the requirements related to large scale industrial production. Furthermore, the fact that the crosslinking of this fluorosilicone composition is based on an SiH/SiVi hydrosilylation mechanism poses problems of stability.
U.S. Pat. No. 5,260,348 also discloses a silicone composition crosslinkable under UV radiation according to a condensation mechanism. Such a composition is of use in the preparation of an antiadhesion coating (release coating), in particular for cylinder head gaskets. This composition comprises an xcex1,xcfx89-diOH POS of the polydimethylsiloxane or polydifluoroalkylsiloxane type, a crosslinking agent of the methyl- or ethyltriacetoxysilane or methyltribenzoxysilane type, and a photoinitiator formed by an onium (iodonium) salt, the counteranion of which is BF4xe2x88x92 or SbF6xe2x88x92. These silicone compositions crosslinkable under UV radiation by condensation have to be subjected to an additional thermal activation in order to obtain reasonable crosslinking times. It is obvious that this complicates the industrial process.
The counteranion with the best performance for crosslinking under UV radiation is SbF6xe2x88x92 but it is found that it exhibits the major disadvantage of being toxic.
It is therefore necessary to observe that, in the current state of the art, there does not exist a crosslinkable silicone composition entirely well suited to the requirements of the specific application, such as impregnation and/or coating of sheet gaskets and in particular cylinder head gaskets. The expected specifications for the impregnation and the varnishing of cylinder head gaskets, which are in particular the ability to render leaktight, the compressibility, the antiadhesiveness, the stability and the ease of use and of preparation, are not satisified by the crosslinkable silicone compositions known to date.
In this state of knowledge, one of the essential objects of the present invention is to reveal a crosslinkable silicone composition for the impregnation and the varnishing of sheet gaskets, in particular of cylinder head gaskets, which overcomes the deficiencies of the prior art and in particular which offer a process for the application and for the preparation/crosslinking of the silicone coating which is fast, which does not require sophisticated equipment or high temperatures, and which makes it possible to obtain a sufficiently antiadhesive varnishing coating possessing suitable mechanical properties.
Another essential object of the present invention is to find a crosslinkable silicone composition for the impregnation and/or the varnishing of sheet gaskets, in particular of cylinder head gaskets, which is stable on storage, which is economical and which does not involve toxic solvents.
Another object of the present invention is to provide a process for the coating of a sheet gasket, in particular of a cylinder head gasket, by impregnation and/or varnishing using a crosslinkable silicone composition, this process having to be easy to employ, to be economical and fast, and to result in a silicone-comprising cylinder head gasket which is antiadhesive and which performs well in the surroundings in which it is used.
Another essential object of the present invention is to provide a sheet gasket and in particular a cylinder head gasket coated and/or impregnated with an antiadhesive and compressible crosslinked silicone exhibiting all the expected specifications set out above as regards xe2x80x9cprocessibilityxe2x80x9d and intrinsic qualities of the material.
Once these objects have been set, it is to the credit of the Applicant Company to have found, after many studies and experiments, that, entirely surprisingly and unexpectedly, it is appropriate to select silicone compositions crosslinkable by the cationic route, preferably under activation by UV radiation, in which the initiator is an onium borate and/or a borate of organometallic cations having the distinguishing feature of having a borate counteranion of an entirely specific nature, in particular in that it comprises a borate atom bonded to at least one phenyl radical substituted by at least one electron-withdrawing group. In addition to this advantageous selection of the initiator, the Applicant Company has isolated a class of POS carrying organofunctional bridging groups (Gfp) crosslinkable by the cationic route, for example of epoxide or vinyl ether type, in which compounds these Gfp groups are present in an amount of at least 0.01 equivalent per kg of POS.
Thus it is that the present invention relates to the use:
for carrying out impregnation(s) and/or for preparing coating(s) which is (are) antiadhesive employed at the engine block cylinder head interface of engines and applied to sheet gaskets, in particular cylinder head gaskets,
of compositions based on at least one polyorganosiloxane (POS) crosslinkable by the cationic route, preferably under UV radiation, and on an effective catalytic amount of at least one initiator salt (PI),
the said use being characterized in that:
(i) the initator salt (PI) is formed by at least one borate of an onium of an element from Groups 15 to 17 of the Periodic Classification [Chem. and Eng. New, Vol. 63, No. 5, 26 of Feb. 4, 1985] or of an organometallic complex of an element from Groups 4 to 10 of the Periodic Classification (same reference),
xe2x96xa1 the cationic entity of the said borate being chosen from:
1)xe2x80x94onium cations of the formula (I):
[(R1)nxe2x88x92Axe2x88x92(R2)m]+ xe2x80x83xe2x80x83(I) 
xe2x80x83in which formula:
A represents an element from Groups 15 to 17, such as, for example, I, S, Se, P of N,
R1 represents a C6-C20 carbocyclic or heterocyclic aryl radical, it being possible for the said heterocyclic radical to comprise nitrogen or sulphur as heteroelements,
R2 represents R1 or a linear or branched C1-C30 alkyl or alkenyl radical; the said R1 and R2 radicals optionally being substituted by a C1-C25 alkoxy, C1-C25 alkyl, nitro, chloro, bromo, cyano, carboxy, ester or mercapto group,
n is an integer ranging from 1 to v+1, v being the valency of the element a,
m is an integer ranging from 0 to vxe2x88x921, with n+m=v+1,
2)xe2x80x94the oxoisothiochromanium cations disclosed in Patent Application WO 90/11303, in particular the sulphonium cation of 2-ethyl-4-oxoisothiochromanium or of 2-dodecyl-4-oxoisothiochromanium;
3)xe2x80x94sulphonium cations where the cationic entity comprises:
3.1. at least one polysulphonium species of formula III.1 
in which:
the Ar1 symbols, which can be identical to or different from one another, each represent a monovalent phenyl or naphthyl radical optionally substituted with one or more radicals chosen from; a linear or branched C1-C12, preferably C1-C6, alkyl radical, a linear or branched C1-C12, preferably C1-C6, alkoxy radical, a halogen atom, an xe2x80x94OH group, a xe2x80x94COOH group, a xe2x80x94COO-alkyl ester group, where the alkyl part is a linear or branched C1-C12, preferably C1-C6, residue, and a group of formual xe2x80x94Y4xe2x80x94Ar2, where the Y4 and Ar2 symbols have the meaning given immediately below,
the Ar2 symbols, which can be identical to or different from one another or Ar1, each represent a monovalent phenyl or naphthyl radical optionally substituted with one or more radicals chosen from: a linear or branched C1-C12, preferably C1-C6, alkyl radical, a linear or branched C1-C12, preferably C1-C6, alkoxy radical, a halogen atom, an xe2x80x94OH group, a xe2x80x94COOH group or a xe2x80x94COOxe2x80x94alkyl ester group, where the alkyl part is a linear or branched C1-C12, preferably C1-C6, residue,
the Ar3 symbols, which can be identical to or different from one another, each represent a divalent phenylene or naphthylene radical optionally substituted with one or more radicals chosen from: a linear or branched C1-C12, preferably C1-C6, alkyl radical, a linear or branched C1-C12, preferably C1-C6, alkoxy radical, a halogen atom, an xe2x80x94OH group, a xe2x80x94COOH group or a xe2x80x94COOxe2x80x94alkyl ester group, where the alkyl part is a linear or branched C1-C12, preferably C1-C6, residue,
t is an integer equal to 0 or 1,
with the additional conditions according to which:
when t=0, the Y symbol is then a Y1 monovalent radical representing the group of formula: 
xe2x80x83where the Ar1 and Ar2 symbols have the meanings given above,
when t=1:
on the one hand, the Y symbol is then a divalent radical having the following meanings Y2 to Y4:
Y2: a group of formula: 
xe2x80x83where the Ar2 symbol has the meanings given above,
Y3: a single valency bond,
Y4: a divalent residue chosen from: 
a linear C1-C12 alkylene residue, a branched C1C12 alkylene residue and a residue of formula xe2x80x94Si(CH3)2Oxe2x80x94,
on the other hand, solely in the case where the Y symbol represents Y3 or Y4, the Ar1 and Ar2 (terminal) radicals have, in addition to the meanings given above, the possibility of being connected to one another via the Yxe2x80x2 residue consisting of Yxe2x80x21, a single valency bond, or of Yxe2x80x22, a divalent residue chosen from the residue cited with respect to the definition of Y4, which is inserted between the carbon atoms, facing each other, situated on each aromatic ring in the ortho position with respect to the carbon atom directly bonded to the S+ cation;
3.2. and/or at least one monosulphonium species having a single S+ cation centre per mole of cation and comprising, in the majority of cases, species of formula: 
in which Ar1 and Ar2 have the meanings given above with respect to the formula (III.1), including the possibility of connecting directly between them only one of the Ar1 radicals to Ar2 according to the way indicated above with respect to the definition of the additional condition in force when t=1 in the formula (II) involving the Yxe2x80x2 residue;
4)xe2x80x94organometallic cations of formula (IV):
(L1L2L3M)q+xe2x80x83xe2x80x83(IV) 
xe2x80x83in which formula:
M represents a metal from group 4 to 10, in particular iron, manganese, chromium or cobalt,
L1 represents a ligand bonded to the metal M via xcfx80 electrons, which ligand is chosen from xcex73-alkyl, xcex75-cyclopentadienyl and xcex77-cycloheptatrienyl ligands and xcex76- aromatic compounds chosen from optionally substituted xcex76-benzene ligands and compounds having from 2 to 4 condensed rings, each ring being capable of contributing to the valency layer of the metal M via 3 to 8 xcfx80 electrons,
L2 represents a ligand bonded to the metal M via xcfx80 electrons, which ligand is chosen from xcex77-cycloheptatrienyl ligands and xcex76-aromatic compounds chosen from optionally substituted xcex76-benzene ligands and compounds having from 2 to 4 condensed rings, each ring being capable of contributing to the valency layer of the metal M via 6 or 7 xcfx80 electrons,
L3 represents from 0 to 3 identical or different ligands bonded to the metal M via "sgr" electrons, which ligand(s) is (are) chosen from CO and NO2+; the total electronic charge q of the complex to which L1, L2 and L3 and the ionic charge of the metal M contribute being positive and equal to 1 or 2;
the anionic entity of the said borate having the formula:
[BXaRb]xe2x88x92
xe2x80x83in which formula:
a and b are integers ranging from 0 to 3 for a and from 1 to 4 for b, with a+b=4,
the x symbols represent:
a halogen atom (chorine or fluorine) with a=0 to 3,
an OH functional group with a=0 to 2,
the R symbols are identical or different and represent:
a phenyl radical substituted by at least one electron-withdrawing group, such as, for example, OCF3, CF3, NO2 or CN, and/or by at least 2 halogen atoms (very particularly fluorine), this being when the cationic entity is an onium of an element from groups 15 to 17,
a pheny radical substituted by at least one electron-withdrawing element or group, in particular a halogen atom (very particularly fluorine), CF3, OCF3, NO2 or CN, this being when the cationic entity is an organometallic complex of an element from groups 4 to 10,
an aryl radical comprising at least two aromatic nuclei, such as, for example, biphenyl or naphthyl, which is optionally substituted by at least one electron-withdrawing element or group, in particular a halogen atom (very particularly fluorine), OCF3, CF3, NO2 or CN, whatever the cationic entity;
(2i) the POS comprises at least one monomer and/or one oligomer and/or one polymer selected:
from compounds comprising at least one (organo) functional bridging group crosslinkable by the cationic route (Gfp) with a heterocyclic nature having one or more electron-donating atoms, such as O, S, N and P,
and/or from those comprising at least one ethylenically unsaturated Gfp group which is substituted by at least one electron-donating atom which increases the basicity of the xcfx80 system, epoxidized POSs and/or POSs carrying vinyl ether groups being particularly preferred as Gfp, and
(3i) the Gfps are present in a proportion (expressed in eq per kg of POS) of at least 0.01, preferably at least 0.10, and more preferably still in a proportion of 0.15 to 2.00.
The present invention therefore proceeds from a novel and inventive advantageous selection of a specific class of silicone compositions crosslinkable by the cationic route, preferably under UV radiation, which is marked out by the nature of its (photo)initiator of the onium borate or borate of organometallic cations type with a borate counteranion of the borophenyl substituted by electron-withdrawing, for example fluorianted, groups type, as well as by a specific POS exhibiting an appropriate level of Gfp functional bridging groups.
Such a selection gives access to an entire series of attractive advantages, namely in particular:
the excellent stability (pot life) of the composition, the change in the viscosity of the latter remaining slight despite the presence of the initiator for several days, indeed even several months, after preparing the composition, provided that the latter is stored with light excluded;
a very good reactivity at ambient temperature;
a low cost price due to the low cost of the starting materials and to the simplicity of the application/crosslinking process for impregnation and/or varnishing;
good flexibility of use, it being possible for the reactivity, the stability, the viscosity and the coefficient of friction of the varnish of the impregnating agent or of the varnish which is non-crosslinked to be easily varied, to a large extent, by varying:
the molor mass of the POS oil carrying Gfp,
the concentration of initiator,
as well as the Gfp/POS molar ratio;
a coating process for the impregnation and/or the varnishing which is simple to employ, which is fast and which does not require sophisticated and expensive equipment and which is therefore, ultimately, productive and profitable;
no need to resort to toxic products in the composition.
It should also be noted that the varnishes or the impregnations obtained on sheet gaskets, in particular on cylinder head gaskets, exhibit a beautiful appearance (absence of bubbles) and satisfy the required qualities of antiadesiveness, of leaktightness and of compressibility. Furthermore, the varnish and/or the impregnating agent which is crosslinked has good mechanical properties, resistance to abrasion as well as thermal behaviour, behaviour towards oils and more generally behaviour towards aggressive products, such as the cooling liquid, which are entirely satisfactory.